Optical Wavelength Division Multiplexed (WDM) systems ideally require passive optical wavelength multiplexers, demultiplexers, and comb filters, all of which ideally should have isolated pass-bands which are flat-topped so as to allow a measure of tolerance in the spectral positioning of the individual signals of the WDM system within these pass-bands. One method of multiplexing or demultiplexing channels in an optical WDM system relies upon the use of multilayer dielectric interference filters. Another relies upon Bragg reflection effects created in optical fibres. A third method, the method with which the present invention is particularly concerned, relies upon diffraction grating effects.
One optical waveguide format that such a diffraction grating can take for wavelength multiplexing/demultiplexing is the format described in EP 0 254 453, which also refers, with particular reference to its FIG. 5, to the possibility of having a tandem arrangement of two diffraction gratings arranged to provide a combined intensity transfer function that is the product of the intensity transfer function of its component diffraction grating 40 with that of its component diffraction grating 42.
An alternative optical waveguide format that such a diffraction grating can take is an optical waveguide grating that includes a set of optical waveguides in side-by-side array, each extending from one end of the array to the other, and being of uniformly incrementally greater optical path length from the shortest at one side of the array to the longest at the other. Such an optical grating, sometimes known as an arrayed waveguide grating (AWG), constitutes a component of the multiplexer described by C Dragone et al., `Integrated Optics N.times.N Multiplexer on Silicon`, IEEE Photonics Technology Letters, Vol. 3, No. 10, October 1991, pages 896-9. Referring to accompanying FIG. 1, the basic components of a 4.times.4 version of such a multiplexer comprise an optical waveguide grating array, indicated generally at 10, whose two ends are optically coupled by radiative stars, indicated schematically at 11 and 12, respectively with input and output sets of waveguides 13 and 14. Monochromatic light launched into one of the waveguides of set 13 spreads out in radiative star 11 to illuminate the input ends of all the waveguides of the grating 10. At the far end of the grating 10 the field components of the emergent light interfere coherently in the far-field to produce a single bright spot at the far side of the radiative star 12. Increasing the wavelength of the light causes a slip in the phase relationship of these field components, with the result that the bright spot traverses the inboard ends of the output set of waveguides 14 as depicted at 15. If the mode size of the waveguides 14 is well matched with the size of the bright spot, then efficient coupling occurs at each of the wavelengths at which the bright spot precisely registers with one of those waveguides 14.
The difference in optical path length between the inboard end of a waveguide 13 and that of a waveguide 14 via adjacent waveguides in the array 10 (the optical path length of a waveguide being the product of its physical length with its effective refractive index) determines the value of the Free Spectral Range (FSR) of the grating for this particular pair of waveguides, being the frequency range over which this difference in optical path length produces a phase difference whose value ranges over 2.pi. radians. Accordingly the single bright spot is produced in the same position each time the optical frequency is changed by an amount corresponding to a frequency difference that is an integral number of FSRs. It can thus be seen that, for optical transmission from any particular one of the set of waveguides 13 to any particular one of the set of waveguides 14, the device of FIG. 1 operates as a comb filter whose teeth are spaced in frequency by the FSR of its grating 10. The propagation distances across the radiative stars themselves contribute to the FSR of any particular combination of waveguide 13 and waveguide 14, and so not all the FSRs are precisely identical.
Movement of the bright spot across the end of the particular waveguide 14, that occurs in consequence of a change of wavelength, results in an approximately Gaussian transmission pass-band for each tooth of the comb filter (or channel of the multiplexer/demultiplexer). For operation in a practical WDM transmission system, a more nearly flat-topped transmission pass-band is generally a requirement in order to avoid excessive uncertainties in the value of insertion loss that the device is liable to provide as the result of tolerances allowed for in the emission wavelengths of the optical sources employed in that transmission system, and to allow for the modulation bandwidth of the signals transmitted in the individual WDM channels. In this context, it may be noted that the drive to narrower channel spacings will typically aggravate this problem because, in general, the tolerances imposed upon the precision of source wavelengths are not tightened in proportion to the narrowing of the channel spacings, and/or the modulation bandwidth tends to constitute a greater proportion of the channel spacing.
An AWG of this kind can be concatenated with a second AWG by optically coupling one of the output waveguides of the first AWG with one of the input waveguides of the second, an example of such an arrangement being described for instance in EP 0 591 042 with particular reference to its FIG. 3. This tandem arrangement provides a combined intensity transfer function that is the product of the intensity transfer functions of its two component diffraction gratings. The response of this tandem arrangement also provides a typically Gaussian fall off in power that is similarly far from the ideal of a flat-topped response.
A known way of providing a measure of flattening of the transmission pass-band of an AWG that is described in U.S. Pat. No. 5,629,992 involves interposing a length of wider waveguide (not shown in FIG. 1) between the input waveguide 13 and the first star coupler 11. This wider waveguide (also known as a multimode interference (MMI), or mixer, section) is capable of guiding, not only the zero order mode, but also the second order mode, both of which are excited by the launch of zeroth order mode power into it from the waveguide 13 by virtue of the fact that the transition between waveguide 13 and the MMI section is abrupt (non-adiabatic). These two modes propagate with slightly different velocities, and the length of the wider waveguide is chosen to be of a value which causes .pi. radians of phase slippage between them. Under these conditions, the field distribution that emerges into the star coupler 11 from the end of the wider waveguide is double peaked. The amount of band-pass flattening thereby occasioned can be expressed in terms of an increase in the value of a figure of merit (FoM) parameter arbitrarily defined as the ratio of the -0.5 dB pass-band width to the -30.0 dB pass-band width. A significant drawback of the mixer section approach to pass-band flattening is that the insertion loss is intrinsically increased consequent upon the mismatch between the size of the flattened field distribution that is incident upon the inboard end of the output waveguide 14 and that of the field distribution of the zero order mode that is guided by that waveguide 14. By way of example, a mixer section supporting the zero and second order modes can be employed to increase the FoM of an AWG from about 0.14 to about 0.30, but this improvement in FoM is achieved at the expense of increasing the insertion loss of the device by approximately 2 dB. Further flattening can be obtained by widening still further the width of the mixer section to enable it to guide a larger number of even order modes, but this introduces yet higher increases in insertion loss. For instance, if the FoM is increased in this way to about 0.45, this is achieved at the expense of an excess insertion loss of approximately 4dB. Additionally, it can be shown that the widening to support the simultaneous guidance and propagation of three or more modes is liable to introduce dispersion problems unless the MMI section could be somehow specially configured to provide the guided modes with a substantially even spacing of propagation constants (.beta.) instead of the uneven spacing typical of most waveguide configurations.
A construction of multiplexer/demultiplexer that also uses a tandem arrangement of optical waveguide gratings, but which is capable of achieving a response that is nearly flat-topped without introducing an excessive insertion loss is described in the specification of WO 98/04944, to which specification attention is specifically directed and its teaching incorporated herein by reference. For the multiplexer/demultiplexer of WO 98/04944 to operate properly as a multiplexer/demultiplexer, it is necessary for the two AWGs of the tandem arrangement to have different values of FSR. Specifically, the FSR of one AWG needs to be made equal to the optical frequency spacing of the channels being multiplexed/demultiplexed, while the FSR of the other AWG needs to be made at least as large as the product of this frequency difference with the number, N, of the channels that the device is designed to multiplex/demultiplex. The FSRs of the two AWGs of the WO 98/04944 multiplexer/demultiplexer must therefore differ by a factor of at least two.